Heparan sulfate (HS) represents a major class of glycans that perform central physiological functions. Heparin, a special class of HS polysaccharides, is an anticoagulant that inhibitss factors Xa and IIa via its ability to modulate the antithrombin (AT) mechanism. However, medicinal administration of exogenous heparin has been shown to elicit heparin-induced thrombocytopenia (HIT) in 2-5% of patients. This response results from a Type II hypersensitivity reaction to heparin-platelet factor 4 (PF4) complexes. Studies have shown that the interaction of heparin and PF4 may, in part, be mediated by charge. Emerging HS and glycosaminoglycan microarray techniques are used to interrogate HS's structure and function relationship to develop novel therapeutic agents. However, availability of HS with specific sulfation patterns has been a limiting factor and impedes the accuracy of HS glycomics studies. Although organic synthesis provides oligosaccharides, these may not fully represent the biological functions of polysaccharides. Here, we present a study for developing an enzyme-based approach to synthesize a polysaccharide library with different sulfation patterns. Using different combinations of biosynthetic enzymes, we synthesized eight unique polysaccharides. We discovered that polysaccharides without the iduronic acid residue displayed strong binding affinity to antithrombin and high anti-Xa and anti-IIa activities. We also used this biosynthetic method to regulate the overall charge of these glycans by enzymatically modulating the degree of sulfation in order to reduce the binding of these polysaccharides to PF4. By employing a biosynthetic scheme, we generated different species of polysaccharides encompassing varying sulfation patterns. We discovered that modulation of the amount of N-sulfation at the beginning of synthesis is the most important motif for decreasing the polysaccharide's propensity for PF4. The anticoagulant activity of these polysaccharide species was maintained although their affinity for PF4 was decreased. This enzyme-based synthetic approach could become a general method for discovering new HS structures with unique biological functions. Advisors/Committee Members: Jones, Courtney, Liu, Jian, Jarstfer, Michael, Lee, Andrew, Lawrence, David, Pedersen, Lee.